Add basic implementation of bytes type, piggybacking on str.

This reuses as much str implementation as possible, from this we
can make them more separate as needed.

tests/basics/bytes.py

@@ -0,0 +1,11 @@
+a = b"123"
+print(a)
+print(str(a))
+print(repr(a))
+print(a[0], a[2])
+print(a[-1])
+
+s = 0
+for i in a:
+    s += i
+print(s)